Transmission of signals (e.g., data, clocks, or other signals) across high-speed chip-to-chip interconnects may take a number of forms. One example illustrating transmission of a signal between high-speed components within a single semiconductor device, or between two devices on a printed circuit board, is represented by the system 10 shown in FIG. 1. In FIG. 1, a transmitter 12 (e.g., in a microprocessor or memory controller 8) sends a signal over a transmission channel 14 (e.g., conductive traces “on-chip” in a semiconductor device or on a printed circuit board) to a receiver 16 (e.g., in another microprocessor or memory 9). The receiver 15 can then send the received signal to a functional circuit 17 within the receiving device 9. For example, receiving device 9 may comprise a Dynamic Random Access Memory (DRAM) integrated circuit, with the functional circuit 17 comprising the memory array of that integrated circuit. One skilled in the art will recognize that typically more than one channel 14 is often present between the devices 8 and 9, forming a bus, but only one such channel 14 is shown for simplicity.
As shown, the devices 8 and 9 and/or the transmitter 12 and receiver 16 are powered by power supply voltages, Vdd and GND (ground). As one skilled in the art will appreciate, digital logic within the devices 8 and 9 will typically be referenced to these power supply voltages, such that a logic ‘1’ bit is represented by Vdd, while a logic ‘0’ is represented by ground or 0 V. In the example shown, the power supply voltage Vdd is equal to 1.2 V in devices 8 and 9, which is a value typical for high-end, modern-day integrated circuits.
It has been recognized that while the transmitting device 8 and receiving device 9 may operate in accordance with a power supply voltage Vdd, it can be beneficial to transmit signals between them on channel 14 having a reduced swing. For example, although both devices 8 and 9 operate with a power supply voltage of 1.2 V, note that the signal being transmitted is referenced to a reduced voltage, Vred, which in the example shown equals 0.4 V. In other words, the signal on the channel 14 swings not across the full power supply voltage range GND to Vdd (e.g., 0 V to 1.2 V) as would be traditional, but instead swings across the reduced voltage range GND to Vred (e.g., 0 V to 0.4 V).
Using a reduced swing to transmit a signal on a transmission channel conserves power. The power consumed by the transmitter 12 in a traditional full swing application is P=C*Vdd2*f*N, where C equals the equivalent output capacitance of, as well as the capacitive loading seen by, the transmitter, f equals the maximum frequency at which switching can occur, and N equals the probability of transition (generally less than or equal to ½ for randomly varying data; but 1 for a clock signal). However, in a reduced swing application, the power consumed by the transmitter 12 equals P=C*Vdd*Vred*f*N, which is less than the traditional application by a factor of Vdd/Vred, or 3 in the example shown.
However, while implementing reduced swing signal transmission is helpful from a power consumption standpoint at the transmitting end of the channel, the system must also employ a receiver 16 capable of receiving such a signal with good reliability. This is problematic, because the reduced swing hampers the receiver 16's ability to quickly and accurately resolve the signal. For example, a traditional operational amplifier or sense amplifier could be used for the receiver 16, in a pseudo-differential configuration, with one input comprising the signal from the channel 14, and the other input comprising a reference voltage (Vref) comprising the midpoint of the reduced voltage, Vred/2, or 0.2V. However, when one considers that the reference voltage may vary (say from 0.15V to 0.25V), and the signal may vary from optimal values (say from 0.4 V to 0.35 V for a logic ‘1’ and from 0 V to 0.05 V for a logic ‘0’), this potentially creates a very small voltage difference window (0.1 V) which the receiver 16 must be capable of resolving. Such a small difference is certainly difficult to resolve in a time frame allowed by the relatively high frequency of the signal. In addition, the relatively low common-mode level of the signal from the channel 14 and the corresponding Vref, falls below the threshold voltage of common n-type devices upon which the pseudo-differential receiver is often based, making such a receiver unable to operate efficiently on the incoming signal. A p-type receiver of the same basic architecture could be employed, as the low common-mode signal levels are suitable for a p-type style receiver, but p-type receivers are notoriously slow, and may not meet the timing requirements of high-speed systems.
Because of the problems using pseudo-differential sensing, one could consider transmitting and receiving both the true and complementary versions of the reduced swing signal, a so-called “fully” differential transmission approach. See, e.g., U.S. patent application Ser. No. 11/972,209, filed Jan. 10, 2008. Such an approach would require a differential receiver to be used, and because the differential data is input into the receiver, the sensing margin would essentially be doubled, allowing for faster, more reliable sensing of the reduced swing signal. However, a differential transmission approach adds complexity and power, and either requires doubling the number of channels 14, or halving the throughput should the same number of channels 14 be retained. Additionally, the “fully” differential receiver would need to be p-type based to cope with the low common-mode signal levels, and as was discussed above, the slow performance of p-type receivers makes them less suitable for the high-speed system.
An alternative would be to let the reduced swing signal start at and come down from the positive supply voltage Vdd (e.g., Vdd-Vred to Vdd). This would shift the common-mode level of the signal higher allowing for an n-type receiver. While this modification would likely perform better in terms of signal integrity, it is a relatively high power solution, as the differential receiver typically requires static DC bias currents and therefore burns power even when no signal is present. This solution would also place undesirable constraints on the system. For example, because the signal swing would be correlated to Vdd, equivalent Vdd levels would be required at both ends of the channel. But in many cases, it is preferable from a power and reliability standpoint to have different Vdd levels at either end of the channel (e.g., when a microprocessor at one end can operate at 1 V while a memory device at the other end requires at least 1.2 V).
Furthermore, the inventors believe the receiver 16 optimally would not merely resolve the received reduced swing signal, but would ultimately boost such signals back to swing levels usable by the receiving device 9, i.e., from 0 to Vdd, i.e., the power supply voltage being used by the receiving device 9. This insures compatibility with the digital logic making up the majority of the remaining circuits on the receiving device 9. The inventors believe such boosting should occur before the signal is captured or latched at the receiving device 9 by a clock signal, such as a clock signal accompanying the reduced swing signal on its own dedicated clock channel. To understand this statement, it is helpful to consider some of the trends in high-speed digital interfaces. In lower speed applications, a forwarded clock not only propagates in parallel with the data being transmitted across the channel, but the clock and data paths are further matched inside the receiving device 9 to insure that any voltage or timing noise impacting either signal will impact both. Careful matching over both paths insures that such noise events cancel out at the point of data capture. With increasing system speeds, however, it has become more difficult to buffer the data as it enters the receiving device 9, and thus matching clock and data paths in the receiving device becomes difficult if not impossible. An alternative method has been to capture data signals immediately as they enter the receiving system, using the forwarded clock which has been distributed out to each of the data ports. This introduces some mismatch in the clock and data paths, and de-correlates the noise between them, reducing the amount of noise cancellation at the point of data capture.
In summary, a reliable, simple receiver 16 useable in a reduced swing transmission scheme is desired, and the inventors realize that such a receiver preferably would be of quick speed; would not involve sensing relying on a Vref; would not involve differential sensing; and would resolve a transmitted signal prior to capture or latching. Examples of such a receiver are disclosed herein.